1. Field of the Invention
The present invention relates to lanthanum metal-oxide superconductors and more particularly to type II, perovskitic superconductors of the LaBa.sub.2 Cu.sub.3 O.sub.y type, wherein y is between about 6.8 and about 7.0.
2. Background of the Invention
A useful superconductor material must possess a number of electronic and mechanical properties. For example, the superconductor must have zero resistance at a relatively high temperature in a variety of conditions. It must also be able to support a large current density without losing its superconductive state. The higher the temperature at which a superconductor achieves zero resistance, the more desirable it is, provided it can support a high critical current density. Of particular interest at the present time are superconductors which are operative at temperatures above the boiling point of liquid nitrogen (-196.degree. C.).
Superconductors can be divided into two rather different types, type I and type II. In type I superconductors, which include most of the metals exhibiting superconductivity, the coherence length, the maximum distance over which two electrons can maintain the coupling that gives rise to superconductivity, is much greater than the magnetic field-penetration depth. In type II materials, the coherence length is much smaller than the field penetration depth and these materials tend to have much higher superconductivity transition temperatures. Most of the current work to develop practical applications for superconductivity has been with type II materials.
One particularly interesting class of type II superconductor, identified as offering the potential of achieving superconductivity at a temperature above the boiling point of liquid nitrogen, comprises orthorhombic crystalline perovskite metal oxide materials which are admixtures of metals selected from groups IB, IIA and IIIB of the Periodic Table. A representative class of these materials is known as "123" and is of the general formula R.sub.1 Ae.sub.2 Cu.sub.3 O.sub.y, wherein R is yttrium or a "lanthanide" metal select from lanthanum, neodymium, samarium, europium, dysprosium, holmium, ytterbium and lutetium, or mixtures thereof, Ae is an alkaline earth selected from calcium, barium or strontium and y ranges from about 6.0 to about 7.0. Lanthanum is a particularly interesting constituent for these materials because (1) it is by far the most abundant of the lanthanide materials found useful in producing superconductive materials, and (2) the relatively larger size of the crystal lattice, as compared to perovskites prepared with other lanthanide materials, significantly extends the range of lattice parameters available to match these materials with particular substrates in applications requiring epitaxed films of high temperature superconductors.
While a number of synthesis processes have been proposed for forming such superconductor materials, they have not proven to be very successful when lanthanum is used as R, because lanthanum tends to replace at least some of the barium in the crystal lattice, while oxygen in the basal planes of the lattice tends to be poorly ordered insofar as superconductive perovskite type lattice structure is concerned. Consequently, the resultant materials have superconductivity temperatures which are below the boiling point of liquid nitrogen, if superconductivity can be achieved at all.
Over the past few years there has been considerable effort to overcome this problem and considerable improvements have been made in the processes used to make perovskitic LaBa.sub.2 Cu.sub.3 O.sub.7 type materials (hereinafter identified as La123). For example, Maeda (Maeda et al. Jpn. J. Appl. Phys, 26, 1368 (1987)) has reported a process for producing La123 samples having zero resistance at 80.degree. K., said process involving first sintering the material at high temperatures in oxygen and then annealing it at temperatures below about 340.degree. C. More recently, Meng (R. L. Meng et al. "Extended Abstracts High Temperature Superconductors II" pp 233-235 April, 1988, Reno, Nev.) has reported the production of La123 materials with superconductive onset temperatures between about 90.degree. and 100.degree. K. and transition widths of 12.degree. K., but the details of the process were not revealed. Wada (Wada et al., Appl. Phys. Lett. 52, 1989 (1988)) has described a process starting with a triple calcining and grinding of the starting material and then sintering this material in an inert atmosphere at temperatures of about 950.degree. C. for 40 hours, followed by slow cooling in a pure oxygen atmosphere, and then annealing in dry oxygen at 300.degree. C. for about 40 more hours to produce materials having zero resistance at about 93.degree. K. Such processes have proven to be expensive, time consuming and/or somewhat erratic in the results achieved.